why do you want this expression to be inline math rather than display?
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David CarlisleMar 14 '14 at 16:51

When you add an underbrace, it increases the depth of the expression so much that there is almost a guarantee of uneven linespacing below it. Because of this, it is a "(not unreasonable) assum[p]tion" (to quote D.Carlisle) for \underbrace to expect it has been used in a display, where that extra vertical space is not a problem.
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DanMar 14 '14 at 18:44

So given that David has told you why it is that way, there are several things you can do with that information. You can create your own macro \tunderbrace that is just like \underbrace but in text style, by altering the existing definition.

Or you, just for fun, create your own version, \Tunderbrace, using stacks: